1. Field of the Invention
The present invention relates to a wiring board equipped with an insulating substrate of aluminum oxide ceramics. More specifically, the invention relates a wiring board in which the surface wiring layer formed on the surface of the insulating substrate is formed of a low-resistance conductor, and the surface wiring layer and the insulating substrate are formed through co-firing.
2. Description of the Prior Art
As the semiconductor elements are integrated ever highly densely in recent years, the semiconductor device equipped with semiconductor elements is generating heat in ever increased amounts. In order to prevent malfunctioning of the semiconductor devices, therefore, it has been urged to provide a wiring board capable of emitting heat out of the device (thermal request).
Operation speed required for the semiconductor devices is also very increasing. To cope with an increase in the operation speed, therefore, it is essential to transmit signals without delay. For this purpose, it has been urged to decrease the conduction loss in the wiring layer of the wiring board, i.e., to form the wiring layer using a low-resistance conductor (electrical request).
As the wiring board mounting a semiconductor element, there has been widely used the one equipped with an insulating substrate of alumina ceramics and having a wiring layer of a high-melting metal such as tungsten or molybdenum formed on the surface of, or inside of, the insulating substrate, from the standpoint of maintaining high reliability in the circuit (wiring layer). In the wiring layer formed of a high-melting metal used for the above-mentioned wiring board, however, it is not possible to decrease the resistance to be smaller than 8 milliohms/.quadrature., and the above-mentioned electrical request is not satisfied.
Recently, furthermore, there has been proposed a wiring board equipped with an insulating substrate formed of glass ceramics that can be co-fired with a low-resistance conductor such as copper or silver. This wiring board is capable of satisfying the above-mentioned electrical requirement. However, the glass ceramics has a thermal conductivity which is as small as several watts/m.multidot.K at the greatest, and makes it very difficult to satisfy the above-mentioned thermal request.
In order to simultaneously satisfy the thermal request and the electrical request, therefore, Japanese Unexamined Patent Publication (Kokai) No. 8503/1996 and 15101/1995 teach the methods of forming a wiring layer comprising copper and tungsten or molybdenum on an insulating substrate of aluminum oxide ceramics having a high thermal conductivity by the co-firing.
According to the technology disclosed in Japanese Unexamined Patent Publication (Kokai) No. 8503/1996, however, the firing is effected at a high temperature which is not lower than 1600.degree. C. in order to increase the density of aluminum oxide. Therefore, tungsten and molybdenum are quickly sintered to form coarsely coagulated particles, whereby the molten copper is separated on the surface of the wiring layer, the surface of the obtained wiring layer is oozed, the wiring layer loses stability in the shape, and the texture in the wiring layer becomes nonuniform causing the wiring layer to exhibit an increased resistance. During the step of firing, furthermore, copper in the wiring layer is diffused in the surrounding ceramics. That is, the diffusion of copper results in a drop in the insulation among the wiring layers, making it difficult to highly densely form the wiring layers having fine patterns on the insulating substrate.
According to the technology disclosed in Japanese Unexamined Patent Publication (Kokai) No. 15101/1995, a conductor layer corresponding to a wiring layer is arranged in the inside of an unfired insulating sheet that will become an insulating substrate, and is co-fired to prepare the insulating substrate. The surface of the insulating substrate is polished so that the wiring layer in the surface of the insulating substrate is exposed. Or, a thin film is formed on the surface of the insulating substrate by deposition or sputtering. Or, a thick film is formed by applying a conducting paste or by firing. Thus, the wiring layer is formed on the surface of the insulating substrate through such steps as a step of polishing, a step of forming a thick film or a step of forming a thin film. An increase in the number of the steps decreases the yield and increases the cost.
Japanese Patent No. 2666744 proposes a wiring board having an insulating substrate of aluminum oxide ceramics, and a wiring layer of copper formed on the insulating substrate by the co-firing. In this wiring board, however, the wiring layer of copper is formed by co-firing requiring the use of a very fine alumina powder having an average particle diameter of from 5 to 50 nm for forming the insulating substrate. Such a fine aluminum powder, however, is very cumbersome to handle and is expensive. Therefore, the wiring board disclosed in the above patent cannot be mass-produced and is not advantageous from the viewpoint of cost, either.